This invention relates to transmission networks and, more particularly, to networks for coupling transmission paths of a type including a transformer.
In transmission networks it is customary to couple a 2-wire path, for example, a subscriber loop, to another 2-wire path or to a 4-wire path via a coupling transformer or a hybrid transformer. It is also customary to provide direct current to the subscriber loop via windings of the transformer. Consequently, the direct current flowing the subscriber loop and, hence, the transformer windings, induces a magnetic flux which tends to saturate the transformer core. As is known, such saturation results in unwanted distortion of alternating current signals, for example, voice being transmitted over the network.
One way of minimizing the affect of the subscriber loop direct current is to employ a transformer having air gaps or a large core. These solutions, however, are undesirable because of the resulting increase in size of the transformer core and, hence, the transformer. The larger transformers are especially undesirable in most circuits in which the transformer is the largest component thereby dictating circuit package dimensions. Use of larger transformers requires more space and, consequently, reduces packaging density which increases costs.
One attempt at eliminating the need to use larger transformer cores and/or air gaps is disclosed in U.S. Pat. No. 3,665,125, issued May 23, 1972. In the disclosed arrangement an additional winding is employed in conjunction with a control transistor to generate a magnetic flux in the core to cancel the flux generated by the direct current flowing in the normal primary windings of the transformer. One notable problem with this prior arrangement is that use of an additional winding necessarily increases both the size and cost of the transformer. Indeed, in most applications the number of turns of the additional winding is equal to the turns of the primary windings. Additionally, this prior flux compensation arrangement is limited to the use of an additional winding to maintain isolation between the primary and secondary windings of the transformer.